Injection molding is a process for converting material from one form or shape to another by expending energy. Typically, material in the raw form of solid pellets is loaded into an injection molding machine wherein the pellets are first converted to a molten state by introducing energy in the form of heat and mechanical shear. Further energy is expended to inject the plasticized material under pressure into a mold having a cavity defining the final shape of the part to be produced while the mold is clamped forcibly closed. Additional energy is used to cool the material within the mold to return it to a solid state. The mold clamp is then opened, the molded part ejected and the mold reclosed in preparation for molding the next part. The laws of physics require that the total energy input to the molding machine equally balance the energy output.
In an hydraulic injection molding machine, energy is input to the machine in the form of electrical energy. Much of this energy is converted into hydraulic flow energy by means of an electric motor drivably connected to a hydraulic pump. The fluid delivered by the pump serves to operate various hydraulic components including electro-hydraulic control valves and hydraulic actuators. The pressure or volume flow demands for hydraulic fluid vary considerably not only from one given machine set-up to another but, also during different phases of the operating cycle of a machine in any given set-up. For instance, a set-up requiring rapid injection of the material into the mold will require a higher volume flow rate than that needed for a set-up calling for slower injection. Also, phases of the machine operating cycle such as the clamp open phase typically require greater hydraulic flows than the part curing phase. While a hydraulic injection molding machine must be capable of supplying whatever maximum fluid pressure and/or flow requirements are needed to meet the machine's maximum rated capacity, significant energy losses have been incurred when operating under conditions where lesser hydraulic fluid pressures and/or volume flow rates are required to operate the machine than the pressure and/or volume flow actually delivered by the pump.
In machines having a fixed-displacement hydraulic fluid pump driven by a fixed-speed electric motor, the pump must be driven to constantly deliver sufficient flow capable of satisfying maximum machine requirements even though the instantaneous hydraulic demand may be significantly lower. The excess flow, i.e., the difference between the actual flow delivered by the pump and the instantaneous demand, is pumped over relief valves. In doing so, energy is wasted. Some of the wasted energy is given off in the form of heat which causes undesirable heating of the hydraulic fluid itself. Further energy must be expended when the fluid temperature rises to a point where active cooling is required to restore the fluid to a suitable operating temperature. In an effort to overcome these problems, there have been various attempts in the prior art to match the output of the hydraulic pump with the demand presented by the rest of an injection molding machine.
One such attempted solution has been to provide a hydraulic injection molding machine having a fixed displacement hydraulic pump driven by an AC motor with an adjustable speed drive such as a variable frequency or inverter type drive. By varying the speed control input to the drive, the motor speed and therefore the pump delivery rate can be varied to more closely approximate the actual hydraulic demand. Unfortunately, the energy savings of such machines are realized at the expense of machine performance. Because the moving components of the motor/pump assembly cannot be accelerated or decelerated very rapidly, the frequency response of the machine is degraded. As a result, molded part quality may be adversely affected due to variations in molding parameters such as shot weight, injection velocity and injection pressure.
The assignee of the present application has marketed a line of hydraulic clamp machines under the trademark VISTA. Those machines utilize a fixed RPM AC motor to drive an axial piston, swash-plate design, variable displacement pump to insure that only the amount of flow necessary to meet load conditions is delivered by the pump thereby reducing energy usage and hydraulic fluid heating. A hydraulic feedback circuit is used to effect either pressure compensation or flow compensation of the pump output on a selectable basis by moving the angle of the swash-plate of the pump to vary its stroke. Since such control action does not require acceleration or deceleration of high inertia rotating components of the motor/pump assembly, response time is significantly improved as compared to the fixed displacement pump/variable speed AC motor machines discussed above. Further improvements in transient response are realized by connecting the pump output to a gas-charged accumulator. The output of the accumulator is connected to a multi-function servo-controlled proportional directional valve which provides closed-loop control of injection velocity, injection pressure, back pressure and melt decompression. Nevertheless, the energy savings realized in VISTA model machines has been less than the savings made possible by the present invention.